A modern radar tracking system is typically comprised of a receiver system and a digital processing system. The receiver system is typically comprised of an antenna, or antenna elements themselves comprising an antenna array, a multi-channel receiver, signal down-conversion, and some analog processing. The digital processing system is typically comprised of high-speed hardware processing and software-based processing.
Data sorting and pulse sorting in particular can be integral to the real-time tracking of radar emitters. Simple pulse-sorters can rely on preselected delay intervals, often the pulse repetition interval (PRI) minus one-half of the peak-to-peak jitter, and acceptance duration testing on pulse length. Tracking pulse sorters would have the delay interval and acceptance duration vary based on the actual accepted pulse intervals. The sorter would be reset after allowing for a predetermined number of missing pulses.
In addition, the duration, frequency characteristics, and amplitude of the current pulse could be compared to a reference or to the previously accepted pulse or an ensemble of previously accepted pulses. Failure to pass these tests can mean that the sorter was no longer locked on to the pulse train. The decision to accept or reject the pulse is typically made after the pulse appears so that one unacceptable pulse may appear at the output before the sorter reacts.
To prevent locking on to a similar signal that occurs during the time between scans, a lockout interval based on the scanning properties of the signal could be included if amplitude measurements were made. Because pulse amplitude (PA) is a strong function of distance to the emitter, it can often easily deinterleave signals that would otherwise be quite confusing. Pulse duration, or pulsewidth (PW), is often used along with amplitude. Pulse duration is less effective as a deinterleaving parameter because many radars are similar in this respect and the measured pulse duration of a particular emitter inherently varies with the amplitude. In addition, multipath causes variation in measured pulse duration values. Angle-of-arrival (AOA) can be measured on a single-pulse basis and is often used in combination with deinterleaving processes. In addition, carrier, or radio, frequency (RF), is a very powerful and commonly used sorting parameter.
Accordingly, in order to sort, associate or reject each signal from the myriad of signals, a sensitive radar tracking system may intercept each instantaneous signal intercepted by the receiver system, which is typically characterized by a set of parameters prior to storage and processing. This characterization provides the information required to associate a set of signals belonging to a particular emitter and to identify that emitter from among other emitters whose signals have been intercepted. The parameters generally measured by the receiver system for a pulsed signal include RF, PA, PW, time-of-arrival (TOA), and AOA. Also, in some systems, polarization of the input signal is measured. Frequency modulation on-the-pulse (FMOP) is another parameter that can be used to identify a particular emitter and also can be used to determine chirp rate of phase coding of a signal using pulse compression. TOA measures are made with respect to an internal clock at the leading edge, and in some cases the trailing edge, of the pulse. AOA measures can be enhanced or replaced by AOA determination processes typically calculated in the software digital processing.
With interferometric devices, it is typical that the amplitude and phase difference for each channel, receiver temperature and instantaneous frequency of every digital sampling point of a valid pulse designated by a unique pulse number be recorded. The parameters measured on a single intercepted pulse are typically stored in a data vector called a pulse descriptor word (PDW) or a “data group.” Multiple PDWs form a set of vectors in parameter space. By matching vectors from multiple pulses, it is possible to isolate those signals associated with a particular emitter.
Deinterleaving can be accomplished through the pulse-by-pulse processing techniques relying on the matching of a number of pulse characteristics (e.g., RF, AOA and TOA) and can benefit greatly from histogram pre-processing approaches. Thereafter, pulse repetition intervals (PRI) can be computed for enhanced emitter characterization.
There remains a need for purposeful pulse delays in a digital signal gating apparatus that can retain leading edges of pulses that would otherwise be rejected due to noise thresholding. U.S. Pat. No. 4,866,314 to Traa discloses a transistor network to be used to delay a digital input signal. U.S. Pat. No. 6,154,497 to Gatherer, et al., discloses a method and system using an oversampled analog-to-digital converter (ADC) together with time adjustments and filters that purport to produce digital signals with lower error than more complex prior approaches. U.S. Pat. No. 5,686,850 to Takaki, et al., discloses a plurality of delay signals from which a particular delayed signal is selected based on phase detection, and through exploitation of a disclosed relationship, a second delayed signal is determined.
Fundamental to advanced forms of deinterleaving is the exploitation of the information contained in the leading edges of accepted pulses. The several embodiments of the present invention have several features, one of which is the ability to record this information in real-time while concurrently testing the acceptability of each of a continual stream of pulses.